It is not clear exactly where this road is located in the Lake Okoboji area, but the fact that it is a long causeway between two bodies of water strongly implies it is the U.S. 71 Highway route between the two lakes.
The L.L. Cook Co., Milwaukee, W
The lakes are described as being distinct entities, but this navigable connection between them indicates they are now one body of water. This card likely dates from the 1940s.